This application relates to infiltrated silicon carbide-based materials, and, more particularly, to such materials wherein the processing is controlled to avoid residual silicon in the final product.
Ceramic materials are inherently brittle and hard. Post-fabrication machining operations are difficult and costly because they usually involve extensive diamond grinding and polishing. It is therefore beneficial to form and process a ceramic article to its final shape to avoid or minimize secondary machining operations. Near net shape forming techniques exist and are put into practice for ceramic articles.
Silicon carbide is one such ceramic material, which may be prepared to its final form. In one fabrication technology designed to yield a stoichiometric silicon carbide material, a preform containing particulate carbon is first prepared. One end of a wick, made of a material that is wetted by molten silicon, is attached to the preform, and the other end left free on a support surface. The required stoichiometric equivalent of solid silicon is placed onto the free end of the wick, and the entire assembly is placed into a vacuum furnace and the furnace is evacuated to an appropriate pressure. The vacuum furnace is heated to a temperature above the melting point of the silicon, so that the silicon melts. The molten silicon is drawn along the wick by capillary forces to infiltrate and wet the carbon particulate preform, where it reacts in situ with the carbon present in the preform to form silicon carbide. Upon cooling, the silicon carbide is present in its as-fabricated, final form.
In a preferred form of this process, the preform also contains fibers such as silicon carbide fibers. The carbon is present as particles interspersed between the fibers of the preform. The infiltrated silicon reacts with the carbon located between the fibers to produce silicon carbide. Upon cooling, the result is a near-net-shape composite material of fibers such as silicon carbide fibers in a silicon carbide matrix.
While this process is operable and has been used successfully in a wide variety of applications, there is room for improvement. In some instances incomplete reaction of silicon has been observed, so that some free silicon remains in the material after cooling to room temperature. Upon reheating to a sufficiently high service temperature, the silicon may remelt and remain in contact with the silicon carbide fibers for a prolonged period of time. The silicon can attack the silicon carbide fibers, resulting in diminished properties of the composite material. Even where no silicon carbide fibers are present in the material, the presence during service of a molten phase within an otherwise solid material is undesirable.
Thus, there is a need for an improved silicon carbide material produced by the silicon infiltration approach. The present invention fulfills this need.